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%I #24 Sep 13 2017 09:53:42
%S 28,52,68,92,124,156,172,244,260,308,327,340,348,356,380,396,404,428,
%T 436,500,508,516,532,580,612,644,660,684,696,724,732,748,764,780,796,
%U 820,836,908,940,980,996,1056,1076,1124,1172,1180
%N Numbers k such that gpf(2^k - 1) - 1 is not divisible by k.
%C For a(n) <= 10^3, all terms are divisible by 4 except 327 = 3*109.
%C Primes p such that 4*p is a term are 7, 13, 17, 23, 31, 43, 61, ...
%C From _Thomas Ordowski_, Sep 04 2017: (Start)
%C If p == +-1 (mod 8) and 2^p - 1 is prime, then 4*p is a term.
%C Conjecture: 4 * A001153(m) for m > 3 is a subsequence.
%C Primes q such that q-1 is a term are 29, 53, 157, 173, 349, 397, ... (End)
%e 28 is a term because gpf(2^28 - 1) == 15 (mod 28).
%Y Cf. A000225, A005420, A006530.
%Y Subsequence of A292199.
%K nonn
%O 1,1
%A _Thomas Ordowski_ and _Altug Alkan_, Sep 04 2017
%E a(10)-a(41) from _Giovanni Resta_, Sep 04 2017
%E a(42)-a(46) from _Max Alekseyev_, Sep 13 2017
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